1. Field of the Invention
The present invention relates generally to a liquid propellant rocket engine, and more specifically to a low pressure pump for a turbopump for the rocket engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Turbopump feed systems are used for high thrust and long duration liquid propellant rocket engines in order to lower the system weight and raise performance over the pressurized gas feed systems. The turbopump fed systems requires only relatively low pump inlet pressures, and thus low propellant tank pressures, while the major portion of the pressure required at the thrust chamber inlets is supplied by the pumps. This saves considerable tank weight, particularly for larger vehicles.
The overall trend is toward higher chamber pressure for liquid propellant rocket engines. The role of the turbopump in the entire system becomes of greater importance, particularly with the high-performance hydrogen-fueled engines. The advantage of the pump-fed over the pressure-fed engines increases as mission velocity requirements increase, and becomes very substantial as orbit-insertion velocities are approached.
The rocket engine designer must ensure that propellants go to the inlet of the pumps at required minimum pressures. The turbopump feed system raises the pressure of the propellants received from the vehicle tanks and delivers them to the main thrust chamber, through ducts and valves, at pressure and flow rates commensurate with rated engine operation. The principal requirements of the rocket engine propellant pump are high reliability, low cost, light weight, stable flow for the required operating range, and long life.
The principal requirements of a rocket engine propellant pump are high reliability, low cost, light weight, stable flow for the required operating range, high efficiency, adequate suction performance, and long life. The relative importance of these factors and their resulting influence on the design will vary depending on the application. The most common used pump types are centrifugal (or radial), axial, or mixed flow pumps. Centrifugal pumps are usually designed with a single stage while axial pumps have multiple stages. Centrifugal pumps can handle large flows at high pressures efficiently as well as economically in terms of weight and size. Thus, almost all of the operational rocket propellant pumps are centrifugal pumps.
A centrifugal pump will accelerate the fluid flow by imparting kinetic energy to the fluid in the rotor and then decelerating, or diffusing, the fluid in the stator. This results in increased fluid pressure head. The rotor assembly usually includes an inducer, an impeller, bearings, and a shaft. The stator assembly consists of a casing with stationary diffuser vanes, a volute with discharge outlets, and seals.
An inducer, an axial flow rotor, increases total pressure of the entering fluid sufficiently to permit non-cavitating operation of the main impeller. An inducer can reduce the pump inlet pressure net positive suction head (NPSH) requirements substantially. The impeller of the centrifugal (or radial) pump basically is a rotating wheel with blades that discharge the flow in a radial direction. The inducer can consist of either a single or a double blade row. The inducer pump is used as low speed boost pumps to raise the pressure sufficiently to permit the main pump to operate at much higher speeds to reduce its size and cost.
A turbine is used to provide shaft power to the propellant pumps and typically derived their energy from the expansion of a high-pressure, high-temperature gas to lower pressures and temperatures. Turbines can be impulse or reaction. Impulse turbines can be either single or multiple staged. Reaction turbines are usually multistage. Impulse turbines are most frequently used for high pressure ratio, low flow applications, Reaction bladed turbines are more frequently used for low pressure ratio, high flow designs.
Several different types of engine cycles are available and can be classified primarily based on where the turbine drive fluid originates and where it is discharged after leaving the turbine. The specific type of coupling between turbine and pumps depends not only upon the propellants being pumped but also on the design of the overall engine system. Various turbopump drive arrangements are known in the art. Where a single pump turbine directly drives both propellant pumps through a common shaft, it can be located either on the shaft end (with back-to-back pump arrangement) or between pumps. Then both pumps and turbine will operate at the same shaft speed. Gear driven turbopump arrangements include the pancake type which uses different reduction gears and is applied where there are speed differentials between pumps and turbine, the offset turbine, with both pumps on one shaft but driven through a gear train; and the single geared pump where one pump is mounted with the turbine on the same shaft, while the other is driven through a reduction gear. Dual-shaft turbopump arrangements with pump and turbine for each propellant on separate shafts include two gas turbines in series, with the discharge gas from the first turbine driving the second turbine, and two gas turbines in parallel, both receiving gas directly from the power source.
Turbopump performance affects the vehicle payload in three ways. 1) Turbopump component weight. Since the turbopump components form a part of stage burnout weight, they directly affect stage burnout. 2) Required pump inlet suction pressure. Required suction pressure directly translates into required main propellant tank pressure level. Suction pressure raised, tank and pressurization system weights increase and thus reduce the stage payload for a given burnout weight. 3) Turbine gas flow rate. For gas generator cycles, the turbine drive gases are usually ejected at a lower specific impulse than the thrust chamber gases. Their flow rate decreases the overall Is (specific impulse) of the engine system and thus for a given velocity increment it decreases the allowable stage burnout weight. For a fixed weight of engines, tanks, guidance, and other equipment, a decrease in allowable stage burnout weight decreases payload weight.
Available pump suction pressure together with the basic pump flow characteristics will determine the maximum shaft speed at which the unit can operate. The higher this shaft speed, the lower the turbopump weight will likely be.